OK, this is with Pro 3 v1 I think (just received it).
1. Load a patch wiith a sequence and make sure it is a 16 step single sequence.
2. Play the sequence.
3. Whilst the sequence is running, press the Arp button and then Seq Modes soft button. You should see "Single" and also "16 Steps" greyed out.
4. Use Soft Knob 2 to change the number of steps to any number below 16. Once done, use Soft Knob 1 to change Seq Mode to "Extended" so it should now run as a sequence of the number of steps you have just selected.
5. It doesn't and keeps playing 16 steps. This happens when play is stopped or running.
6. To get it to work, you have to change to Extended before adjusting the number of steps.
Currently, it's not possible to change the number of steps in any single sequence without changing it to extended first. Can't it just be made to use the number of steps as shown in the display right from the start?
Hope that helps.
LN